10.5 Breakdown of polysaccharide: chitin

Chitin, in which the repeating unit is the same as that in cellulose except
that the hydroxyl group at C-2 is replaced by an acetamido group (Fig. 2), is
the second most abundant polymer on Earth as it occurs in the exoskeletons of
arthropods and, of course, in fungal cell walls. Polysaccharides which contain
amino sugars or their derivatives are called mucopolysaccharides.
Chitin is degraded by chitinase, a glucan hydrolase which
attacks the β1→4 glycosidic bonds, eventually producing the disaccharide
chitobiose which is then converted to the monosaccharide N-acetylglucosamine
by chitobiase (Seidl, 2008). Chitinase may also be involved in fungal wall
synthesis (see Chapter 6).
Several chitinases, which are glycoside hydrolase family enzymes, are produced
by fungi and can have different substrate-binding site structures and are
obviously specialised for different functions. Derivatives of chitin and
chitosan have found many commercial uses from medicine to cosmetics and dietary
supplements, so fungal chitinases have biotechnological applications, also
(Seidl, 2008; Hartl et al., 2012).

Fig. 2. Structural formulae of chitin and its
constituents. Chitin structure is similar to that of cellulose (Fig. 1),
except that acetylamino groups replace the hydroxyl groups at carbon
position 2 of the glucose molecule. Approximately 16% of naturally
occurring chitin units are deacetylated. Modified from Moore, 1998.